LayoutElement.preferredWidth

Description

The preferred width this layout element should be allocated if there is sufficient space.

The preferredWidth can be set to -1 to remove the size.

using UnityEngine;
using System.Collections;
using UnityEngine.UI; // Required when using UI elements.

public class ExampleClass : MonoBehaviour
{
    public Transform MyContentPanel;

    //Sets the flexible height on on all children in the content panel.
    public void Start()
    {
        //Assign all the children of the content panel to an array.
        LayoutElement[] myLayoutElements = MyContentPanel.GetComponentsInChildren<LayoutElement>();

        //For each child in the array change its LayoutElement's preferred width size to 250.
        foreach (LayoutElement element in myLayoutElements)
        {
            element.preferredWidth = 250f;
        }
    }
}